Any reference to background art herein is not to be construed as an admission that such art constitutes common general knowledge in Australia or elsewhere.
Ultraviolet light (UV) absorbing or screening compounds have found use in a range of applications where protection from the sun's harmful UV rays is desirable. This includes their use in glass and lens coating, paints and materials including fabrics as well as, perhaps most notably, in sun screen formulations to protect the skin of the user from damage caused by UV radiation.
UV radiation can be subdivided into three bands; UVA at 315-400 nm; UVB at 280-315 nm; and UVC at 100-280 nm. UVC is almost completely absorbed by stratospheric ozone and so it is UVA and UVB that present the main risk to people and materials subjected to prolonged exposure.
Amongst the most active natural UV absorbing compounds are the mycosporine-like amino acids (MAA's) examples of which are known with a peak absorption either in the UVA or UVB range and absorption coefficients comparable to those of synthetic sunscreens. There has therefore been considerable focus on the isolation and characterisation of naturally occurring MAA's as well as strong interest in the generation of active derivatives and analogues thereof.
U.S. Pat. No. 5,637,718 (the 718 patent) describes a range of MAA analogues as UV absorbing compounds based on a cyclic enaminoketone core. While a number of these compounds showed useful absorption in the UVB range none of them showed a useful broad range of absorption extending into the UVA region. Indeed, all but two of the compounds synthesised demonstrated maximum absorption within a narrow band of 305-308 nm. This is largely as a result of the limited variance of substitutions around the cyclic enaminoketone core. All but one of the compounds in the 718 patent are substituted on the ring nitrogen with only a simple alkyl chain or, in a single example, a cycloalkyl ring. Further, at the 6-position of the enaminoketone ring no substitutions were made at all. Only a single substitution with a methyl group was proposed at the 2-position of the enaminoketone ring and, again, it does not appear as if this compound was, in fact, synthesised. While some range of substitution was provided for at the 3-position, all compounds had a carbonyl group attached directly to the ring at this position and all but three of the proposed substitutions were with straight chain alkyls.
The simplest compounds disclosed in the 718 patent tend to have an absorbance maximum (λmax) of 307 nm which can be attributed solely to the enaminoketone chromophore. This has not been modified to any real extent by the pattern and nature of substitutions presented in the 718 patent and so the value of the proposed compounds as even somewhat broad spectrum UV absorbing agents is minimal.
It would therefore be desirable to provide for UV absorbing compounds with a greater range of variance in the absorbance maximum to provide a compound or suitable combination of compounds which may afford greater UV protection when formulated for use in a sunscreen composition, a coating composition, or the like.